We investigate the
fatigue resistance of chemically cross-linked polyampholyte
hydrogels with a hierarchical structure due to phase separation and find that the details of the structure, as characterized by SAXS, control the mechanisms of crack propagation. When
gels exhibit a strong phase contrast and a low cross-linking level, the stress singularity around the crack tip is gradually eliminated with increasing
fatigue cycles and this suppresses crack growth, beneficial for high
fatigue resistance. On the contrary, the stress concentration persists in weakly phase-separated
gels, resulting in low
fatigue resistance. A material parameter, λtran, is identified, correlated to the onset of non-affine deformation of the mesophase structure in a
hydrogel without crack, which governs the slow-to-fast transition in
fatigue crack growth. The detailed role played by the mesoscale structure on
fatigue resistance provides design principles for developing self-healing, tough, and
fatigue-resistant soft materials.